The invention relates to a three-phase a.c. motor for operation with a three-phase mains supply.
Three-phase a.c. motors with three single- or multiple-strand windings are customarily powered by a three-phase mains supply in such a way that they can be turned on and off. A known arrangement for this purpose is to dispose, between the three-phase mains and the terminals that connect the motor to the mains, two- or three-phase electromechanical switches, relays or contactors by means of which the electrical connection between the three-phase mains and the motor is made or broken. Instead of electromechanical switching elements, it is becoming increasingly common to use electronic switching elements. With the latter, contact abrasion is avoided. The electronic switching elements allow the amount of current that flows to be controlled by gate terminals. As in the case of electromechanical switching elements, at least two electronic switching elements are required. Ordinarily these consist of triacs, alternistors or a combined system in which each triggering circuit is connected to two thyristors in an antiparallel arrangement.
To reduce line and radio disturbances associated with turning the three-phase motor on and off, and to protect the electronic switches, additional components are added to the circuitry, such as RC elements, recovery diodes and varistors. both the electromechanical switches and the electronic switching elements are customarily separated spatially from the three-phase motor, being disposed in switch cabinets or boxes. This arrangement takes up extra space, in addition to that required for the motor itself. Furthermore, additional material is required to construct the switch cabinet or switch box.
Another known feature is the provision of a mechanical relay (contactor) for each phase in the mains supply line, or also of relays switching onto the neutral point. Such arrangements make the cable construction considerably more elaborate and space-consuming.
When electronic switching elements are used to turn the three-phase motor on and off, the total number of switching and peripheral elements is quite large. As a result, considerable costs are incurred in both manufacture and maintenance of the switching arrangements.
The object of the invention is to disclose a three-phase a.c. motor that is turned on and off by electronic switching elements, such that the overall space required and the number of electronic components are reduced in comparison to the known three-phase a.c. motors and electronic switches, and the reliability of operation is improved.